The concomitants model can be tested through cross-sectional studies comparing persons who are currently depressed, persons who have recovered from depressive episodes, and healthy controls. An even better approach is to conduct longitudinal studies assessing individuals when they are in a depressive episode and again after they have recovered. If personality measures are abnormal during depressive episodes but not after recovery, it would suggest that they are concomitants of the depressed state. Multilevel analyses can also be used to separate personality variance into trait and state components and to test whether state variance is associated with concurrent measures of depression (e.g., Duncan-Jones et al. 1990).
